We plan to continue our long term research involved in the characterization of specific steroid binding proteins for developing model systems to study, first the chemical nature of androgen, estrogen, progestin and glucocorticoid binding sites and, second, the role which these proteins play in the overall mechanism of action of steroid hormones. The chosen models are the Sex Steroid-Binding Protein (SBP) of human serum which we have recently purified (Biochemistry 14, 957, 1975), baboon serum SBP, rabbit serum SBP, rabbit serum CBG (corticosteroid-binding globulin), and the intracellular androgen receptor of rat ventral prostate. Purification in large quantities of the three serum proteins as well as experiments leading to a large-scale purification procedure for the androgen receptor will be carried out with the affinity chromatographic methods established in our laboratory. Our present main effort in characterizing the binding site of human SBP will be extended to the other 3 serum proteins. The experiments will involve chemical modification using both "group specific" reagents and affinity labels containing either photoreactive or haloacyl and diazonium groups. This approach will result in the identification of the amino acid side chains in the binding site responsible for the specific absorption of the steroid on the protein surface. The availability of these pure proteins will permit the preparation of specific antibodies to study the physiological role of SBP. These will be injected into rabbits and androgen responsive tissues will be examined morphologically. Dramatic histological change should result if the levels of "free" testosterone increase as a result of systematic removal of plasma SBP caused by the antibody. In conclusion, the comparison of these various specific proteins will not only result in our further understanding of the chemical factors controlling specificity of steroid binding, but will also serve to establish animal models to study function.